The present invention relates to an aluminum base material with an aluminum alloy deposited on a portion of the aluminum base material to provide a hard facing. The invention relates particularly to a hard facing deposit on aluminum pistons which may be subjected to high operating temperatures, e.g., 600.degree. F.
In most internal combustion engines, the gap between a piston and its respective cylinder wall is sealed by at least one piston ring that is carried by the piston. Specifically, the piston has an annular piston ring groove, and the piston ring is disposed partly in the groove so that it moves with the piston, as the piston reciprocates. The piston ring has surfaces which engage the cylinder wall and surfaces which engage the walls of the piston ring groove in which the ring is located.
Typically, pistons are formed of a base material which is predominantly an aluminum-silicon alloy. Aluminum-silicon alloy is used because it is light weight, and has good wear characteristics, high heat conductivity and low, relatively stable thermal expansion properties at elevated temperatures. As an example, one aluminum alloy which is commercially used as a base alloy for heavy duty diesel engine pistons includes, by weight:
______________________________________ Silicon 10.5%-11.5% Copper 1.5%-2.0% Manganese 0.4%-0.9% Iron 0.7% maximum Zinc 0.4% maximum Magnesium 0.5%-0.9% Impurities 0.8% maximum Aluminum balance ______________________________________
Many aluminum-alloys which are used as a base material for a piston do not have a high degree of hot hardness, i.e., the ability to resist deformation at high operating temperatures. In heavy duty diesel engines, the piston ring groove may be subjected to contact with the piston ring at temperatures as high as 600.degree. F. At such elevated temperatures, most metals, and particularly aluminum-silicon alloys, lose a significant amount of their ability to resist deformation. Moreover, the high pressures which exist in the combustion chamber can apply considerable forces to parts of the piston ring groove, either directly or through the piston ring. If the hot hardness of the walls of the piston ring groove is not enough to resist deformation when subjected to high temperatures and pressures, then the piston ring may not properly seal the combustion chamber. Blow-by (i.e., flow of combustion gases past the piston ring) may occur, lessening the engine's compression ratio and adversely affecting the operation of the engine.
Various techniques have been utilized to minimize deformation of the walls of a piston ring groove. One method is to cast or forge an aluminum base alloy around a ring of harder material such as Ni-resist (a term used in the metallurgical arts for a nickel-containing cast iron), and machine a piston ring groove out of the ring. This technique can increase the hot hardness of the piston ring groove, but it also has some undesirable characteristics. One undesirable characteristic is that the casting or forging operation may not produce a good metallurgical bond between the Ni-resist and the base alloy. Without a good metallurgical bond, the heat conductivity between the material that forms the piston ring groove and the base alloy would not be good, thus impeding the overall ability of the piston to conduct heat away from the area around the piston ring groove.
Another technique which has been suggested for increasing the hot hardness of a piston ring groove is disclosed in U.S. Pat. No. 3,285,717. According to the patent, an aluminum hard facing alloy is welded to the piston, and a piston ring groove is formed at least partly in the weld deposit. The aluminum hard facing alloys disclosed in the patent would include, by weight, 12-30% silicon, 10-30% copper, 2-6% manganese and 0-6% iron.
The alloys disclosed in U.S. Pat. No. 3,285,717 are indicated as having a hot hardness ranging from 45 BHN to 139 BHN at 600.degree. F. Applicant believes those values to be the hot hardness values for the alloys in an "undiluted" state, i.e., before they are welded to the base alloy. However, in welding a commercially available form of the alloys of U.S. Pat. No. 3,285,717 to the commercial aluminum base piston alloy disclosed above, the hot hardness of the hard facing alloy, even before welding, was found to be well below the level applicant was trying to achieve (i.e., 100 BHN at 600.degree. F). During the welding process, the hard facing alloy was diluted by the base material, and there was a significant decrease in hot hardness of the weld deposit. Specifically, an aluminum hard facing alloy consisting essentially of 20% silicon, 15% copper, 4% manganese, 2% iron, and balance aluminum was welded to the piston base alloy set forth above. The weld alloy, in an undiluted state, showed a hot hardness of 63.7 BHN at 600.degree. F. After being welded to the piston base alloy, the weld deposit showed a hot hardness of about 50 BHN at 600.degree. F. When piston ring grooves were machined into weld deposits of the hard facing material, significant deformation of the ring groove walls was experienced at high temperatures.
Applicant, and applicant's assignee, believes that for heavy duty diesel piston applications, the hot hardness of a weld deposit should be substantially improved over the hot hardness properties experienced above. Specifically, applicant believes the weld deposit should have a hot hardness of at least about 100 BHN at 600.degree. F., and should preferably have a hot hardness of considerably above 100 BHN at 600.degree. F. (i.e., 150 BHN or more), to resist deformation adequately when subject to the operating temperatures and pressures in a heavy duty diesel engine.